Many technologies have been used in the past for detection of underground objects such as pipes, power lines, gas lines, etc. These technologies include nuclear, acoustic, gravitational, magnetic and electromagnetic. Electromagnetic devices include infrared, microwave, low-frequency magnetic, and eddy currents. Both active and passive detection systems have been used. Nuclear systems have inherent safety disadvantages, and radar/acoustical, shortwave-longwave or electromagnetic induction systems also have their drawbacks. Sonic systems have the disadvantage of being dependent upon good soil contact.
A wide variety of metal detectors have also been used to locate buried "treasure", for geophysical exploration, for law enforcement purposes, etc. These types of detection devices generally operate to create a near field of continuous wave electromagnetic forces about a central inductive coil. When a metallic object is brought within the field, an impedance change occurs, resulting in the objects detection. These type of detectors are sensitive to variations in the soil giving rise to false signals. Many continuous electromagnetic wave type detectors are typically constructed having a transmit coil mutually coupled with a receive coil. The requirement of mutually coupled coils inhibits the use and application of these type of devices. Because the geometry of the coils are critical for operation, the device must be constructed of rigid members to maintain precise relative coil placement. These type devices are very sensitive to slight jarring or impact which may cause coil movement rendering the device inaccurate.
Efforts to improve detector sensitivity have also been directed to the use of pulsed eddy current detectors in which a pulsed magnetic field is directed toward a target and induces eddy currents in conductive targets. Voltages induced by the decay of the eddy currents are thus detected. In these devices, mutual coupling is not required between the transmit and receive coils.